1. Field of the Invention
The invention relates to a device for powdering printed sheets with powder applicator devices, which are switchable back and forth between two operating states and which, in a first of the operating states thereof, direct to a given destination a free stream of carrier gas carrying entrained powder. The invention also relates to a sheet-fed printing press having a delivery for delivering the printed sheets to a pile station by sheet grippers revolving during operation, and having a device by which powder is distributable to the sheets being transported in the delivery, and also a sheet-fed rotary printing press having a device for indirectly powdering the printed sheets.
A device of the foregoing general type has become known heretofore, for example, from the published Japanese Patent Document JP Hei 5-28634 (U), wherein respective powder nozzles are described as being connected to a supply line that is closable by an electromagnetic valve. Closing and opening of certain supply lines is performed by a controller, which triggers the electromagnetic valves and has an arrangement for specifying the sheet size. The device is provided in the delivery of a sheet-fed printing press and is supposed to prevent the distribution of powder beyond the sheet edges which are oriented in the sheet travel direction. Thus, during delivery of the printed sheets, as a function of sheet size or format, some of the supply lines must be kept closed and the remaining ones must be kept open.
With a device for powdering printed sheets heretofore known from the published German Patent Document DE 40 40 227 A1, for example, it is to a certain extent possible in particular to prevent powder from being deposited on press parts of the delivery of a sheet-fed printing press which are located within the aforementioned edges of the sheets. This heretoforeknown device succeeds in this by not adding the powder to the aforementioned carrier gas steadily but only at a predetermined rhythm or cadence, so that powder application can be limited to those time segments when a particular sheet is moving past the powder applicator devices. To that end, in a chamber containing a bed of powder and communicating with a jet pump, a cloud of powder is created in the aforementioned cadence by intermittently making the surface of the powder bed swirl up; this cloud is then aspirated by the jet pump and admixed with a carrier gas flow passing through the pump, and then blown jointly with this flow onto the respective sheet with the powder applicator devices connected to the outlet of the jet pump. The powder applicator devices thus dispense a steady gas stream during operation, and this stream then carries entrained powder at the aforementioned cadence.
In practical use for powdering printed sheets, which move past the powder applicator devices at the cadence of the sheet-fed printing press, the time intervals during which the gas stream is supposed to be free of powder are many times shorter than those in which it is supposed to carry entrained powder. Especially for the relatively high cadence frequencies that are usual in modern sheet-fed printing presses, it proves to be problematic, with the heretoforeknown device, to keep the gas stream free of powder during the aforementioned brief time intervals.